1. Field of the Invention
The present invention relates to an active-type distance measuring apparatus capable of avoiding erroneous measurement even in a case where the projected light does not properly project on an object or in a case where the object has a distribution of the optical reflectance.
2. Related Background Art
For auto focusing apparatus for an active-type camera, there have been proposed various devices based on the trigonometric principle, and, for avoiding erroneous measurement encountered in such active-type auto focusing apparatus by eclipse of the projected light or by the contrast in an object, the Japanese Patent Laid-open Application No. 63-235909 discloses an apparatus provided with plural position detecting optical systems, receiving the reflection light from the object with plural photo sensor units and performing the position detection by averaging the detected positions of the received light in such plural positions, thereby improving accuracy of measurement. A modification of such conventional apparatus will be explained in the following with reference to FIG. 8.
This distance measuring apparatus is composed of a light projecting unit 46, a first photosensor unit 47 and a second photosensor unit 48 provided on both sides by sandwiching the light projecting unit 46, and a calculating unit (not shown). The light projecting unit 46, for projecting light of a specified wavelength to an object O (49), is composed of an infrared light-emitting diode 51 and a projecting lens 52. The first photosensor unit 47, for receiving the light projected from the light projecting unit 46 and reflected on the object 49 for forming a first light image on a light-receiving face, is composed of a first light-receiving lens 53 and a first position detecting photosensor element 54. The first light-receiving lens 53 is positioned parallel to a film plane 50, in such a manner that it is separated by a baseline distance B from the optical axis .gamma. of the light projecting lens 52 and its optical axis .alpha. is parallel to the optical axis .gamma. of the light projecting lens 52.
The second photosensor unit 48, for receiving the light, projected from the light projecting unit and reflected by the object O, at a position opposite to the first photosensor unit 47 with respect to the light projecting unit, for forming a second light image on a light-receiving face, is composed of a second light-receiving lens 55 and a second position detecting photosensor device 56, in a similar manner as in the first photosensor unit 47. The second light-receiving lens 55 is positioned symmetrical to the first light-receiving lens 53 with respect to the light projecting lens 52, and the second position detecting photosensor device 56 is similar to the first photosensor device 54.
By receiving the reflection light from the object O in a position as shown in FIG. 8, an optical image of the object, lacking a side thereof having a shorter object distance, is formed on the first position detecting photosensor device 54, while an optical image lacking a side thereof having a longer object distance, is formed on the second position detecting photosensor device 56.
If the light-receiving lenses 53, 55 are mutually identical in the lens characteristics (focal length etc.) and the first and second position detecting photosensor devices 54, 56 have the same photoelectric conversion characteristics, the photocurrent ratio of the first and second photosensor devices when the received optical images are not eclipsed can be represented as I.sub.A1 /(I.sub.A1 +I.sub.B1)=I.sub.A2 /(I.sub.A2 +I.sub.B2), (I.sub.A1 =I.sub.A2, I.sub.B1 =I.sub.B2) based on the similarity of triangles, as the first and second photosensor units 47, 48 are positioned symmetrically with respect to the optical axis of the light projecting unit 46.
In determining the variation in the output of the first position detecting photosensor device 54 is represented by (I.sub.A1 +m1)/(I.sub.A1 +m1+I.sub.B1 -m1), while that of the second position detecting photosensor device 56 is represented by (I.sub.A2 -m2)/(I.sub.A2 -m2+I.sub.B2 +m2). In such state, the ratio of the summed photocurrents of both photosensor devices becomes (I.sub.A1 +m1+I.sub.A2 -m2)/(I.sub.A1 +m1+I.sub.B1 -m1+I.sub.A2 -m2+I.sub.B2 +m2)=I.sub.A /(I.sub.A +I.sub.B) (since m1=m2). The object distance L can be determined from this result, based on the known trigonometric processing.
However, the embodiment shown in FIG. 8, requiring the first and second light-receiving lenses in mutually symmetrical positions with respect to the light-projecting lens, necessitates a large space in the application to a camera or the like. Thus the camera itself inevitably becomes larger and is restricted in design in consideration of the limitation in the physical arrangement.